Automobile-based, cellular telephones generally comprise a handset and transceiver unit mounted inside the automobile with the transceiver unit coupled to an external antenna structure through a coaxial transmission cable. The antenna structure is normally externally mounted to a glass automobile window, such as the rear windshield. The antenna structure includes a mono-pole antenna connected to a flat coupling plate which lies against the outside surface of the windshield. Mounted on the interior surface of the windshield and opposite the antenna coupling plate is another flat coupling plate supported by a plastic or metal housing. The transceiver cable is electrically coupled to the inside coupling plate. The two parallel plates and windshield glass form a capacitive circuit which allows cellular telephone signals to be transmitted and received through the glass and between the antenna and transceiver via the cable.
As with most circuits operating at radio frequencies or RF, there is an impedance mismatch in cellular telephone systems between the impedance of the transceiver cable and the impedance of the coupling plates and antenna. The impedance mismatch causes a power reflection at the connection point between the plates and the cable, and a reflected standing wave results. The reflected power reduces the efficiency of the system, interferes with the telephone signals passing through the system and creates stray radiation within the automobile. To improve the operation of cellular telephones, a coupling network is usually placed in-line between the coupling plates and cable to provide an impedance match between the antenna and plates and the cable thus reducing the reflected power.
Generally, the transceiver cable is a coaxial cable having an inner conductor and an outer ground foil covering and is connected between the coupling network and the cellular telephone transceiver. The cable end opposite the transceiver is connected to a coaxial connector having a center conductor and an outer shell to mate with the inner conductor and outer ground foil of the cable. The connector center conductor and shell are, in turn, electrically coupled to the coupling network. The coupling network is also electrically connected to the interior coupling plate and through the glass to the outer plate and antenna. Thereby, coupling networks provide impedance matching between the coaxial transceiver cable and the antenna, and should ideally operate to ensure efficient and generally interference-free transmission and reception of the cellular telephone signals.
While currently available coupling networks provide a certain degree of electrical impedance matching between the transceiver cable and the antenna, many of the commercially available antenna coupling networks do not operate as well as desired. For example, some do not provide sufficient impedance matching. Consequently, there is an undesirable amount of reflected RF energy on the transceiver cable which degrades the quality of the telephone signal, and results in radiating undesirable RF energy into the interior of the automobile. Further, other matching networks are not readily tunable to operate over the three currently-existing cellular telephone frequency bands. These bands are 824-896 MHz for the U.S., 895-960 MHz for Europe and 806-866 MHz for the U.S. Special Mobile Radio or SMR market.
One proposed solution for preventing stray radiation from the coupling network has been to enclose the coupling network in an electrically conductive metal housing which is electrically connected to the connector shell to ground the housing. The housing acts as a so-called "counterpoise" to maintain the coupling network electrically "cool" and reduce the reflection and undesired radiation that results. However, use of a counterpoise requires a formed metal housing which is expensive to manufacture, thus increasing the cost of the system.
Further, the metal housing becomes part of the electrical circuit which may affect performance of the coupling network depending upon its placement relative to the metal of the vehicle. The metal housing also provides a source of potential radiation and reception problems should a loose connection within the housing accidentally contact the housing. An alternative has been to provide a shield near the coupling network that is grounded to the metal of the vehicle about the window. The grounded shield has eliminated some of the problems encountered with use of a counterpoise, but has also led to increased costs and manufacturing complexities.
Therefore, it is desirable to eliminate the cost and other drawbacks associated with a metal housing or shield. To this end, another proposed cellular telephone antenna system uses an electrically closed-loop transformer structure for coupling the antenna to the transceiver cable. The closed-loop transformer is already electrically "cool" and thus has minimal reflections so that both the counterpoise and the grounded shield may be completely eliminated. However, the transformer structure is not without its own drawbacks. In particular, the rigid, metal preform of the transformer structure has a fixed dimension and thus is limited in its tunability and operability over the currently available cellular telephone frequency band ranges. Furthermore, the rigid metal preform is somewhat fragile and susceptable to movement or bending during installation and use which modifies its electrical characteristics and thus degrades its operation.